Trim the crusts from the bread. Roll out the bread with a rolling pin until thin and pliable, then spread with an even layer of cream cheese. Top with a thin layer of smoked salmon, then the watercress.
Roll up into 4 long sausages. Leave to set in the fridge for at least 15 mins or up to 4 hrs.
Using a sharp knife, cut each roll crossways into 4 to make bite-sized pinwheels. Arrange on a platter to serve.